Play with and extend a scripting language. This might be a little old school, but for example the Tcl source is considered to be very clean C. You can download

Play with and extend a scripting language. This might be a little old school, but for example the Tcl source is considered to be very clean C. You can download the source, implement your own command via shared library in C, or simply check out how commands like [open] or [expr] work by digging through the source. I believe the same applys to Perl and maybe Python.